This invention relates to inverters having a transformer-coupled commutating circuit, and more particularly to thyristor single phase and polyphase inverters essentially having a single commutating pulse generator that is transformer-coupled to the power circuit and controlled to effect commutation, one at a time, of all the load current carrying thyristors.
The present invention offers solutions to the following problems that can occur in other types of inverter circuits. In each case the statement of the problem is followed by prior art solutions with their disadvantages.
1. The load current that can be commutated is generally proportional to the main dc supply voltage, so that it is impractical to adjust the dc voltage in order to regulate the ac output voltage. Also, when the dc voltage varies about a fixed nominal value, the minimum voltage determines the size of the capacitor. While it is possible to couple, by transformer action or otherise, an independently controlled commutating pulse into the main circuit having adjustable dc voltage, other techniques generally require a substantial number of accessory components, increasing the complexity and expense. 2. At least one commutating capacitor per pair of main thyristors is normally required. A single commutating capacitor can be shared by the several legs of a polyphase inverter, but prior art methods require at least one auxiliary thyristor for each main thyristor, plus several more auxiliary thyristors.
3. In circuits using auxiliary thyristors to generate the commutating impulse, at least one auxiliary device for each main thyristor is usually necessary. Other inverters having a transformer-coupled commutating circuit with one auxiliary thyristor per pair of main devices have encountered difficulties involving trapped energy.
4. When the ac load is capacitive or regenerative, the commutating pulse may be redundant, but it is generally necessary to generate a high current pulse in order to reverse the charge on the commutating capacitor to be ready for the next commutation. Chopper-type commutating circuits can be controlled to avoid redundant commutations, but are more complex, and transformer-coupled circuits with this capability again have problems with trapped energy.
5. Redundant commutations can result in a rise of output current into overload conditions, but the rise cannot be halted until recovery is completed, which may be long in some cases. Some prior art circuits have a very short redundant commutating time, however the present inverter need have no delay at all.
6. The magnitude of the commutating impulse must be sized to extinguish the worst-case peak overload current even though the load may be small. Multi-level commutating pulse generators are conceivable but prohibitively complex and expensive when a separate pulse generator is required for every pair of main thyristors.
7. Waveshaping the commutating pulses to improve the efficiency of operation usually requires many additional components in a polyphase inverter and is not practical. The drawbacks in paragraph 6 apply to waveshaping as well.
8. Most commutation circuits involve inevitable losses that are dissipated in damped oscillations or otherwise. A known commutation circuit without such losses uses a high voltage auxiliary thyristor for every main device and an expensive magnetic component.
9. Some circuits require high voltage transformer windings and auxiliary diodes to recover energy trapped in reactive elements after commutation. Prior art approaches involve greater complexity for avoiding, minimizing, recovery, or dissipation of trapped energy.
10. Most inverters have only two levels of output voltage. The addition of a third (zero voltage) output level can be accomplished using inverse-parallel coasting thyristors across the load, but this technique has not been employed often in the past because complex auxiliary circuitry was needed to commutate the coasting thyristors.